There are several applications where signals have to be amplified, for example in switched or clocked circuits like analogue digital converters or switched capacitor filters. In many cases, an amplifier providing high gain and at the same time consuming low current is needed, especially for driving on-chip capacitive loads.
Gain boosting is a method widely used to achieve high gain. This technique relies on the use of an auxiliary amplifier which works in parallel to a main signal path comprising a main amplifier. The auxiliary amplifier boosts the effective output impedance and thereby the gain of the main amplifier. In one existing implementation, the main amplifier can comprise a field effect transistor. The auxiliary amplifier can comprise an operational transconductance amplifier with high output impedance. An input of the auxiliary amplifier is coupled to a source terminal of the main amplifier. An output of the auxiliary amplifier is coupled to the gate terminal of the main amplifier. A signal to be amplified is supplied to the gate terminal of the main amplifier. The auxiliary amplifier regulates its output to exactly the same voltage regardless of the input signal seen by the main amplifier. This process of regulation can go on for the entire time available for completion of settling. The output impedance of the main amplifier is thereby enhanced. An amplified signal is provided at the drain terminal of the main amplifier.
The principle of gain boosting is described in detail in K. Bult and G. J. G. M. Geelen, “A fast-settling CMOS op amp for SC circuits with 90-dB DC gain,” IEEE J. Solid-State Circuits, vol. 25, pp. 1379-1384, December 1990.
One possibility to realize low current consumption in amplifiers is by using dynamic biasing. Dynamic biasing techniques rely on changing a bias current flowing through an output branch of an amplifier with respect to the strength of an input signal. This reduces the current consumption when there is less signal activity and increases the current consumption in case of high signal activity. In one existing implementation, a biasing control unit is coupled to the control input terminal of the main amplifier. The signal to be amplified is also fed to an input of the biasing control unit. The biasing control unit generates a control signal proportional to the input signal seen by the main amplifier. The output of the biasing control unit is therefore always optimized for the instantaneous input signal seen by the main amplifier. In order to accomplish its task well, the biasing control unit has to react immediately to instantaneous changes of the signal applied to the main amplifier and track these variations. Therefore, the output of the biasing control unit should have low impedance.
A detailed description of dynamic biasing can be found in R. Castello and P. R. Gray, “A High-Performance Micropower Switched-Capacitor Filter,” IEEE J. Solid-State Circuits, vol. SC-20, no. 6, pp. 1122-1132, December 1985.
The schemes of gain boosting and dynamic biasing are inherently in conflict as both methods operate directly on the output branch of the main amplifier. To achieve high gain and low current consumption, a composite amplifier could be built by cascading a conventional gain boosted amplifier with a dynamically biased amplifier. However, this leads to two or more gain stages, one of which is dynamically biased. This complicates the compensation of the overall amplifier, often requiring performance to be sacrificed. What is more, current consumption is optimized only for the stage which is dynamically biased, whereas the gain boosted stage still operates while constantly consuming maximum power. More stages also imply larger quiescent power consumption.